Speed Continuously Changes Up and Down

1. Check actuator linkage for proper operation. If satisfactory, proceed to step 2. If not satisfactory, adjust or replace actuator linkage.
2. Check continuity of circuits 147, 148 and 149. If continuity exists, proceed to step 3. If continuity does not exist, repair or replace wiring as necessary.
3. Ensure that all system vacuum hoses are tightly connected and are free of cuts, cracks and kinks.
4. Disconnect hose between check valve and servo, at servo end. Apply 18 inches of vacuum to open end of hose. If vacuum can be pumped and held, proceed to steps 5 or 6. If vacuum cannot be pumped or held, replace check valve.
5. On Thunderbird SC models, disconnect hose between aspirator check valves and manifold check valve at manifold check valve end. Apply 18 inches of vacuum to open end of hose. If vacuum can be pumped and held, proceed to step 6. If vacuum cannot be pumped or held, replace aspirator check valves as necessary.
6. On all models, check speedometer cable for proper routing, ensure no sharp bends or binding exists. If cable operation is satisfactory, proceed to step 7. If cable operation is not satisfactory, service or replace speedometer cable.
7. Perform tests on servo. If servo operation is satisfactory, proceed to step 8. If servo operation is not satisfactory, replace servo.
8. Perform tests on speed sensor. If satisfactory, proceed to step 9. If not satisfactory, replace speed sensor.
9. Check for proper operation of vacuum dump valve. If satisfactory, proceed to step 10. If not satisfactory, service or replace as necessary.
10. Replace amplifier with known good amplifier. If system operation is satisfactory, replace amplifier. If system operation is still not satisfactory, ensure all system electrical connectors have proper contact.